The present invention relates to a device for insulating handles from vibration which would, otherwise be transmitted to the handle, in such machines and cause a vibration during the operation, e.g. chain saw, hand breaker for soil engineering or the like.
In these machines which cause or produces vibration, such as chain saws, a vibration acceleration as large as 3 to 40 G is inconveniently transmitted to the handle, so that the workers who handle the machine often suffer from Raynaud's disease. For this reason, an adequate counter-measure for isolating the vibration has been longed for, as is well known.
Conventionally, various countermeasures have been taken, among which the most popular one is to connect the handle to the vibration source, such as engine and chain drive, through a medium of a number of cushioning rubbers. However, this counter measure still falls short of overcoming the above stated problem completely, mainly for the following reasons. Namely, the damping component of the rubber cannot insulate the handle from the vibration especially when the frequency of the vibration is high, as in the chain saw. In addition, the freeness of design is inconveniently restricted due to the nature inherent in the rubber. At the same time, the vibration-isolation characteristic is soon changed due to aging, so that the vibration-isolation performance is deteriorated shortly. Further, the support of the saw is made unstable because of the presence of a number of cushion rubbers, and the power transmission from the handle to the saw is made uncertain, so as to deteriorate the cutting performance of the saw. Furthermore, the cushioning rubber, which is inherently not so strong, is likely to be broken during operation, to incur an unexpected accident.
It is therefore a principal object of the invention to overcome above stated problems or drawbacks of the prior art by providing an improved device for isolating or insulating the handle from vibration in vibration making machines which has a substantial freedom of design to facilitate the optimum design, is less likely to suffer from aging, has a good directionability of the saw and a safe transmission of human force through the handle when the saw is stopped or operated at a low frequency of vibration, is capable of performing a thorough vibration isolation for vibrations at a high frequency range, and small-sized and light-weight with a simple and compact construction.
A reference shall be made here to another type of conventional vibration insulating machine. More specifically, in the so-called a type vibration absorbing device having a spring K interposed between a vibration source 15 and an insulated section m as shown in FIG. 2a, the amplitude ratio at the section m with respect to the vibration source 15 is as small as zero, as will be seen from the characteristic curve a, when the frequency of the vibration is sufficiently high. This feature is quite advantageous in the vibration isolation. However, unfortunately, there is a peak of the resonance point .sqroot.k/m in the region of low frequency of vibration. Thus, in the course of acceleration after starting, there is a point at which the amplitude of the vibration from the source 15 is inconveniently amplified and transmitted to the section m. This often places the machine dangerously out of order.
There has also been proposed a b type vibration absorbing system in which a damper c is interposed between the vibration source 15 and the insulated section m, in parallel with the spring K. This arrangement allows the peak of amplitude ratio to be limited to 1:1. However, at the high frequency range, the vibration is inconveniently transmitted to the section m through the damper c. Further, since the damper relies upon oil, the kinematic coefficient of viscosity is changed by two times by a change of temperature by 10.degree. from the normal ambient temperature, so as to cause a fluctuation in the damping coefficient. In addition, it is necessary to take a specific measure to prevent the oil from leaking out of the damper. However, this measure is usually incompatible with the elimination of mechanical frictional contact, so that the design of the device has been rendered highly difficult.
It is therefore a second object of the invention to overcome above stated problems. To this end, a low-pass characteristic is preferred in which, as shown in FIG. 1, the peak around the resonance point in the low frequency range is suppressed so as to make the amplitude ratio 1:1, thereby to realize the so-called stiff condition of the machine, i.e. the state in which the vibration source 15 and the section m are connected to each other rigidly, whereas the vibration is isolated thoroughly in the high frequency range. Further, the damping coefficient of the damper is made adjustable to afford an outstanding damping effect. In addition, the device should be free from frictional mechanical contact of its parts, easy to handle and attractive in price.